Air/fuel mixing in jet flames

Abstract

The paper examines eight diverse regimes in which fuels can mix and react with air. These comprise: (i) Lifted subsonic; and (ii) supersonic jet flames, with (iii) and without (iv) cross flows; (v) Rim-attached flames; (vi) Early Downwash flames; (vii) Downwash-attached jet flames; and (viii) Fire Whirls.Correlations of characteristics within these regimes are principally in terms of a dimensionless Flow Number, U*, Cross Flow Reynolds number, Rec, and, for Fire Whirls, a dimensionless Critical Velocity, CV. Boundaries of seven of the eight regimes are identified, through plots of U*, against Rec, and of the eighth through a plot of CV against U*. The circumstances of transitions between regimes are identified. The study involves a variety of CH4 cross flow flame measurements, in a wind tunnel. Cross flows can initially create a small lee-side flame downwash, due to the depression in pressure. With increasing fuel flow this might extend 1.3 m downwards from the horizontal tip of the vertical burner. Jet flames can attach to the downwash, which can become significant above Rec ≈ 2000. More extensive downwash might further delay blow-off. Regime boundaries are constructed on the U*/Rec diagram covering lifted flames, early downwash, and downwash-attached flames. The most powerful flames tend to be lifted, choked, flames, with cross flow, and fire whirls. Combustion becomes less efficient at high Rec and low U*, although CH4 was efficiently reacted.Experimental values of the ratio of fuel to air velocity, u/uc, of CH4 flames ranged between about 10 and 30 for lifted flames, and between 0.3 and 3.6, at blow-off, for rim-attached flames. The latter comprise an important category, often intermediate between lifted flames and downwash-attached flames.